Field of the Invention
The present invention relates to a digital audio signal processor and to the control of signal parameters such as gain in such a processor.
Description of the Prior Art
Whilst the present invention may be applied to the control of parameters other than gain, for simplicity and clarity of description, it will be described herein with reference to gain.
In an audio signal mixer, for each output channel there is a plurality of input channels each having at least one manually operated control for controlling gain (or some other parameter). Digital mixers operate on sampled and digitised signals sampled at a rate S1 greater than the Nyquist rate such as 44.1 KHz or 48 KHz. In a digital signal processing channel, gain is controlled by multiplying the digital signal sample values by numbers representing the desired gain using digital multipliers. The desired gain values are set by manually adjusted gain controls.
There is proposed herein a digital audio signal mixer in which a set of manually adjustable gain controls are linked to a digital signal mixer by a control processor (e.g. a computer) and which samples the gain controls. The large number of gain controls are sampled at a rate S2 which much less than the sampling rate S1 of the digital signals because there is a large number of such controls.
The present invention is concerned with the situation in which a relatively low sampling rate of gain (and/or other) controls would result in the gain (and/or other transfer characteristic) of an audio signal processor changing in steps at a rate at which an undesired artifact (i.e. an article effect produced by the manner in which the processor processes signals) which would be audible in the processed audio signal. One example of such an artifact is known as xe2x80x9czipper noisexe2x80x9d.
According to the present invention there is provided a digital audio signal processor for processing digital audio signals having a first sampling rate S1, the processor having a multiplicity of manually adjustable controls for setting desired parameters of signals to be processed, means for sampling the setting of each control at a second sampling rate S2 less than the first rate S1 to determine the settings of the said controls, and means responsive to the sampling means for applying the sampled settings to the signals, wherein for each control the applying means determines the difference of successive samples of setting and applies, to the signal subject to control by that control, increments of setting each increment being a predetermined fraction 1/n of the said difference at a rate nS2 which is n times the said second sampling rate S2.
The rate nS2 is less than or equal to S1. Preferably nS2 equals S1. Preferably n is an integer and more preferably is an integer power of two. Preferably n is fixed.
Thus by incrementing e.g. the gain by fractions 1/n of the gain change set by the manual control at a rate nS2 , audible artifacts are reduced.
According to an embodiment of the present invention, the signal processor is a mixer for 1-bit signals. An embodiment of such a mixer comprises an nth order (where n is greater than or equal to 1) Delta Sigma Modulator (DSM) having a first input for receiving a first 1-bit signal, a second input for receiving a second 1-bit signal, a quantizer for requantizing a p bit signal to 1-bit form the requantized signal being the output signal of the processor, a plurality of signal combiners including a first combiner for forming an integral of an additive combination of the product of the first signal and a first coefficient and of the product of the second signal and a second coefficient and of the product of the output signal and a third coefficient, at least one intermediate combiner for forming an integral of an additive combination of the product of the first signal and a first coefficient and of the product of the second signal and a second coefficient and of the product of the output signal and a third coefficient and of the integral of the preceding stage, and a final combiner for forming an additive combination of the product of the first signal and a first coefficient and of the product of the second signal and a second coefficient and of the integral of the preceding stage to form the said p bit signal which is requantized by the quantizer.
The combiners of the signal mixer operate on 1-bit signals and so coefficient multiplication is performed as 1-bit multiplication avoiding the need for p bit multipliers which are uneconomic.
Furthermore the DSM also provides noise shaping.
The first and second coefficients define zeroes of the input signal transfer function and maybe fixed or variable, but the third coefficients define poles of the input signal transfer function and are fixed.
If the first and second signals applied to the DSM are produced by unsynchronized sources, synchronisation means are required so the bits of the signals are in phase synchronism at the DSM.